For many decades, self-propelled combine harvesters have been used by farmers to harvest a wide range of crops including cereals, maize and oil-seed rape. Typically, a combine harvester cuts the crop material, threshes the grain therefrom, separates the grain from the straw, and cleans the grain before storing in an on-board tank. Straw and crop residue is ejected from the rear of the machine.
Threshing is typically carried out by one or more threshing cylinders which rotate within a drum or cage. Combines of the conventional or hybrid threshing type comprise a threshing cylinder which rotates on a transverse axis wherein the crop material passes tangentially under the cylinder. An axial-type threshing system includes one or more rotors having longitudinally-aligned rotation axes wherein the crop material is fed into a front end and is conveyed in a spiral path around the rotors. Combines having a transverse threshing system include a threshing cylinder having a transverse rotation axis wherein the crop material is fed tangentially at one side and is conveyed in a spiral path around the rotor across the width of the machine.
Grain and chaff separated in the threshing process falls under gravity through a grate onto an underlying conveyance system that passes the grain and chaff to a cleaning unit. The conveyance system typically includes a grain pan which is driven in an oscillating manner to convey the grain and chaff rearwardly to a rear edge from where the grain and chaff falls under gravity into the cleaning unit (often termed ‘cleaning shoe’).
The remainder of the crop stream from the threshing process is conveyed rearwardly from the threshing apparatus into separating apparatus. In a conventional system the separating apparatus typically includes a plurality of straw walkers which ‘walk’ the straw over a grate to the rear of the machine while the separated grain falls through the grate. In pure axial systems the separating apparatus comprises an extension of the one or more axial rotors wherein the separated material falls through a grate provided by the rotor cages.
The material falling through the grates of the separating apparatus is caught by an underlying separator pan (or ‘return pan’) which is also driven in an oscillating manner to convey the material forwardly to a front edge from where it falls under gravity to combine with the grain collected from the threshing apparatus. The straw by-product from the separating apparatus is ejected from the rear of the combine.
The cleaning unit of most combines operates according to a well-established process in which grain and chaff cascading down from the thresher and separator pans is subjected to an airstream created by a fan. The airstream blows the lighter chaff and dust rearwardly and out of the combine whilst the heavier grain falls onto and through a chaffer and one or more cleaning sieves before being conveyed to the grain tank.
The speed of the airflow is chosen so as to maximise the percentage of chaff, and indeed any material other than grain (MOG), removed from the crop-stream whilst minimising the percentage grain loss from the rear of the machine.
Developments in threshing and separating technology of recent years have not been matched by an increase in capacity of the cleaning unit. The bottleneck presented by the cleaning unit therefore inhibits utilisation of the full potential of modern separating technology.